vfs: provide function to get internal file information by fd

sys/include/vfs.h

@@ -882,6 +882,21 @@ int vfs_normalize_path(char *buf, const char *path, size_t buflen);
  */
 const vfs_mount_t *vfs_iterate_mounts(const vfs_mount_t *cur);
 
+/**
+ * @brief   Get information about the file for internal purposes
+ *
+ * @attention   Not thread safe! Do not modify any of the fields in the returned
+ * struct.
+ * @note        For file descriptor internal usage only.
+ *
+ * @internal
+ * @param[in] fd    A file descriptor
+ *
+ * @return  Pointer to the file information struct if a file with @p fd exists.
+ * @return  NULL, when no file with file descriptor @p fd exists.
+ */
+const vfs_file_t *vfs_file_get(int fd);
+
 #ifdef __cplusplus
 }
 #endif

sys/vfs/vfs.c

@@ -864,6 +864,16 @@ const vfs_mount_t *vfs_iterate_mounts(const vfs_mount_t *cur)
     return container_of(node, vfs_mount_t, list_entry);
 }
 
+const vfs_file_t *vfs_file_get(int fd)
+{
+    if (_fd_is_valid(fd) == 0) {
+        return &_vfs_open_files[fd];
+    }
+    else {
+        return NULL;
+    }
+}
+
 static inline int _allocate_fd(int fd)
 {
     if (fd < 0) {